The integrity of the assembly obviously depends on the clamping action of the 4 bolts
keeping the two halves together. If the bolts become loose the unit will leak and
should the bolts fail, the unit will separate with catastrophic results.
In practice, if the unit is mounted in the vertical plane (as is normal), and if
seawater leaks or drips from the hose connection, it will accumulate on the horizontal
upper flange, which will always be wet.
This environment can lead to stress corrosion cracking of the stainless steel in
the area between the head of the clamping bolts and the Marlon surface of the flange
which is wet, salty and oxygen-free. The result of this corrosion mechanism is for
the heads to eventually drop off the bolts, thus destroying the clamping force which
keeps the unit together. In the limit, the two halves of the unit will separate and
the valve will fall apart.
Case Study (Al Shaheen)
Al Shaheen is an aluminium alloy hull launched in 2001. She is fitted with 8 such
seacocks in 1½ “, 1” and ¾” sizes, all fitted in the vertical plane. These are sold
in the UK as “Patay” seacocks and were supplied by Pump International in Falmouth.
In most cases, at some stage, there has been some slight leakage of seawater from
the pipework above the unit which has deposited on the upper flange and caused corrosion
of the clamping bolts.
In 2008 (7 years after launch) an inspection of the clamping bolts revealed that
at least half were badly rusted and in 6 of the total 32 bolts (20%), the head fell
off when touched. One valve was in a very dangerous condition with 3 of the 4 bolt
heads rusted through. All 32 bolts have now been replaced with new and bedded in
place with lanoline under the heads to exclude seawater from the bolt head/Marlon
Pump International were not helpful when this problem was discussed with them by
telephone. They did agree to supply new bolts but, in fact, failed to do so even
after a reminder in writing and return to them of sample failed bolts.
• The corrosion mechanism described above represents a serious threat to the integrity
of this type of seacock.
• Clamping bolts should be inspected at least annually. If evidence of corrosion
is detected around the bolt heads, those bolts affected should be withdrawn and replaced.
Single bolts on any valve may be withdrawn and replaced whilst afloat, if necessary,
provided that the remaining 3 are known to be intact and tight. If there is any doubt
about the condition of the other bolts, a safer procedure would be to have the vessel
hauled out and work carried out ashore.
• This type of seacock is an excellent choice for metal boats but the clamping bolts
must be maintained in good condition.
• Pump International are not a responsible supplier and are not to be relied on.
A non-metallic seacock is an attractive proposition for owners of metal boats as
there is no possibility of galvanic corrosion in an item which is critical to the
integrity of the boat. The normal pattern is a ball valve with a body formed in two
halves from glass-reinforced polypropylene (in the USA referred to as Marlon) with
a Delrin (engineering nylon) ball. The Marlon halves are machined internally to accept
the Delrin ball, which turns easily in a low-friction, plastic-to-plastic contact
surface. Each half of the body incorporates a thick flange on the extremity and,
when assembled, the two halves are sandwiched together with the ball on the inside.
The assembled unit is clamped together with four stainless steel bolts and nuts.
With the non-metallic (plastic) material kept in compression by the bolts, the assembled
unit is very strong. It is also corrosion-free (on the inside), low friction, and
requires no lubrication. It is almost impossible for scale or grit from the seawater
to get into the contact area between ball and valve body.
The assembly is drilled and threaded top and bottom to incorporate inlet and outlet
attachments. On a metal boat the normal arrangement is to have a tubular metal spigot
welded to the hull, threaded at its inner end; the seacock is then screwed onto the
spigot. A plastic hose nipple is then normally screwed into the threaded hole on
the upper face of the seacock to accept plastic piping and hose clips.